Cultured complex communities of attached algae, sometimes referred to as algal turf systems, have been used to manage water quality and to scrub a variety of nutrients or contaminants from marine, estuarine, and fresh water. In particular, such algal communities do not demonstrate the sensitivity and instability characteristics of monocultures of algae, and thus are useful in the removal of nutrients, such as phosphorus, and pollutants from wastewater. U.S. Pat. No. 4,333,263 describes the use of algal turf systems containing attached algae communities to primarily metabolically remove carbon dioxide, nutrients and other pollutants from wastewaters; U.S. Pat. Nos. 4,966,096 and 5,097,795 describe equipment for carrying out this function; and U.S. Pat. No. 5,851,398 describes a method for removing pollutants with algal turf scrubbing systems by precipitation of the pollutants onto and/or into the cells or cell walls of an algal turf growing in a flowing column of water, all of said patents are hereby incorporated by reference in their entirety. Algal turf systems are also known to increase or enhance the production of algal biomass. Algal turf systems containing cultured communities of attached algae have also been described in U.S. Pat. Nos. 5,527,456; 5,573,669; 5,715,774; 5,778,823; 5,851,398; and 6,572,770, all of which are herein incorporated by reference.
When scaled-up, algal turf systems may be used to restore impaired rivers, bays and lakes. For instance, algal turf systems have been successfully scaled-up for nutrient removal from point-source, nonpoint source and open surface waters. In addition to its successful use in the removal of nutrients (such as nitrogen, phosphorus and carbon dioxide) and other contaminants, the literature also illustrates the successful use of algal turf systems, such as the commercial Algal Turf Scrubber® (ATS), for the injection of oxygen into a wide variety of waters. For instance, in Adey et al., “Phosphorus Removal from Natural Waters Using Controlled Algal Production,” pp. 29-39, Restoration Ecology (March 1993), the author discusses the use of ATS to scrub the nutrient phosphorus from agricultural wastewaters. This article, and the above-identified patents, discloses the role of algal turf to remove phosphorus both through (1) metabolic uptake and (2) the physical removal of phosphorus attached to organic particulates. In these references, particulate trapping, i.e., physical removal, is accomplished within the web of algal filaments enhanced by the mucilage production of blue-green algae and diatoms.
Algal turf systems are often characterized by a basal component of algal filaments self-attached to a screen, such as a plastic screen. The screen functions as a growing surface for the algal turf. An epiphytic community of filaments and cells are attached to the basal filaments and bulked-up with the trapping of mostly algal unicells by the built up meshwork of algae. Typically the basal filaments are green algae, such as Cladophora and Rhizoclonium in fresh water; red algae, such as Polysiphonia and Herposiphonia; and brown algae, such as Ectocarpus and Giffordia in both estuarine and sea waters.
In previous floways using algal turf systems, filamentous green algae have been principally used to produce considerable algal communities or algal biomass. In particular, when accompanied by extensive natural energies, such as currents or wave oscillations, the algal biomass have sustained significant levels of productivity. In some cases the algal biomass has been used as an animal feed and fertilizer. The use of algal turf systems have also been expanded to aquaculture, the treatment of tertiary sewage system, and agricultural canal amelioration of nutrients. In addition, algal turf systems have been successfully used in the production of commercial quantities of fish, such as Tilapia. 
Under some water treatment conditions, filamentous diatoms become the primary basal support filaments in algal turf systems. This is the case in those situations where the water being treated lacks natural periphyton and is heavily dominated by filamentous diatoms. In such cases, the filamentous diatoms overwhelm the green and red basal filaments. In green, red or brown filamentous algae, in which the cell walls of adjacent cells are an integrated structure, the filaments have considerable tensile and shear strength. These filaments are able to withstand the flow and surge of water on an algal turf floway during the entire 7-21 day typical harvest cycle even when heavily encrusted with epiphyles. Slough or filamentous breakage and loss only become serious at the end of the harvest cycle which is seasonally determined. In the short term, individual cells of green, red or brown filamentous algae can die without compromising the integrity of the green filament.
Unlike green, red and brown algal filaments whose often massive cellulosic wall is continuous from cell to cell with no break, diatom filaments have an entirely different structure. Filamentous diatoms have their cells attached to each other by much weaker mucous links or thin silica threads. When filamentous diatoms dominate the algal turf basal filaments, frequent breakage and random slough occurs resulting in turf loss before the end of the harvest cycle.
Some filament loss may also be attributable to the substrate used to date in algal turf systems. The traditional substrate in an algal turf system, such as an ATS, is a 2-dimensional plastic screen. Several varieties of plastic screen have been employed including extruded high density polyethylene (HDPE) of 3×5 mm mesh, and woven multifilament nylon and nylon/HDPDE mesh. The dominant diatom communities that occur on some river algal turf systems quickly attach to the 2-dimensional plastic screens but their filaments constantly “shear-off” in the moderate energy environment of an algal turf system, producing a lower standing crop and ultimately lower water remediation capabilities and by-product biomass.
The filament loss exhibited from diatoms limits efficiency during harvesting of the algal turf. Typically, harvesting is effectuated by scraping or vacuuming directly on the drained screen following full biomass buildup. Further, the presence of diatoms negatively affects the productivity of those sloughed filaments and cells. In situations where diatom filaments form the primary structure of the algal turf, estimated filament loss to slough is as high as 50%. While some of this loss may be captured in slough mesh bags or removed by centrifugation, these processes result in increased operations and maintenance costs. In addition, sloughed algal biomass may be captured via outflow filtration systems; however this method results in loss of system productivity. Overall, the lesser shear strength of diatom filaments in the moderate energy environment of the algal turf system increased slough rate, reduced expected productivity by 25-50% and reduced nutrient removal by 10-30%.
Improvements in the use of algal turf systems, especially when used with diatom filaments, are therefore needed to increase harvesting and productivity of water treatments where primary basal support filaments are filamentous diatoms.